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» PCB & ASSEMBLY Source: Indium Corporation Profile #1 vs. Profile #2 Solder paste particle size We experimented with varying the solder paste particle size. The stencil thickness of 0.004“ and Profile #1 were used. The results of this indicate that there appears to be a slight reduction in void area with small solder paste particle sizes going from Type 4 to 4.5 to 5; however, there is a significant increase in voiding area percent with Type 3 paste. Source: Indium Corporation The void area percentages for Profile #1 and Profile #2. Note the dramatic reduction with Profile #2 Zusammenfassung Obwohl es viele Faktoren gibt, die beim Lötprozess zu Lunkern führen, können durch die Anwendung eines Standardprozesses, wie er hier beschrieben wird, mehrere Parameter eingestellt werden, um Lunker zu minimieren. Résumé De nombreux facteurs peuvent causer l’apparition de creux à la soudure, mais ce problème peut être évité grâce au réglage de plusieurs paramètres d’un processus standard, comme expliqué ici. The voiding area percent for selected solder pastes for Profile #1 and Profile #2. Note that Profile #2 not only reduces the void percentages, but significantly reduces the standard deviation or spread in the data Source: Indium Corporation Резюме Хотя существует множество факторов, приводящих к образованию пустот в процессе пайки, использование описанного здесь стандартного процесса позволят настроить некоторые параметры для сведения пустот к минимуму. The 0.004“-thick stencils exhibit more than 4% more void area than 0.005“ stencils Source: Indium Corporation 36 EPP Europe » 11 | 2023
Source: Indium Corporation The 0.004“-thick stencils exhibit more than 4% more void area than 0.005“ stencils Source: Indium Corporation There appears to be a slight reduction of void area with smaller particle sizes in the solder paste from types 4 to 4.5 to 5. Only the difference between type 3 and the other sizes is statistically significant The top X-ray images show poor area voiding percentages greater than 30%, while the bottom photos show voiding of less than 10% Source: Indium Corporation Void percent area as a function of solder pastes. Note the significant difference between pastes, some as low as 5% voids and others around 45% Source: Indium Corporation Solder pastes We performed a final experiment in which we evaluated the effect of different solder paste flux vehicles on voiding. Three PCBs with 12 QFNs each were used for each solder paste, for a total of 36 QFNs per paste. The results were striking, as seen in Figure 11. Some pastes produced about 5% voids, while others produced as much as 45%. These results were not only surprising, but also very encouraging. While the generation of voids is a complicated process with many variables, selecting the solder paste alone, with a standard assembly process can assure voiding area levels of less than 10%. Conclusions Although the factors that generate voids are many, by adopting a standard process as described, several parameters can be set to minimize voiding. The strongest determinant in the assembly process is to have a hotter peak temperature reflow profile. This factor alone reduced voiding area percent from an average of about 22% to less than 8%. Although finer solder particle size reduces solder voiding, the result was only statistically significant by going from Type 3 to Type 4 or finer. Type 3 pastes produced more than 10% more void area than Type 4 or finer. Using a stencil thickness of 5mils reduced voiding about 4% on average as compared to 4mil stencils. The greatest surprise, however, was how significant solder paste formulations were in reducing voiding. The better solder pastes significantly reduced voiding to less than 6% void area, whereas the worst paste produced greater than 45% voiding. Productronica, Booth A4.309 www.indium.com To be continued... The second part of this paper, in which Flanagan, Belfield, and Hotvedt detail the additional investigations they have undertaken to identify and refine void reduction strategies will be published in our April 2024 issue. EPP Europe » 11 | 2023 37
